Skeletal myogenesis is a tightly regulated process requiring coordinated changes in expression of a large number of genes allowing proliferating myoblasts to differentiate in to myotubes (Shieh, 2013). This process is altered in many different myopathies; among them, Duchenne muscular dystrophy (DMD), where the regenerative capacity of skeletal muscle precursors cells (satellite) is lost. Their decreased ability to differentiate into mature and functional myotubes leads to progressive muscle weakness with chronic degeneration.
Although there are a large number of functions attributed to the endocannabinoid system, very little is known about its function in the skeletal muscle cell regeneration and differentiation. Recently, it was found that the endocannabinoid 2-AG inhibits differentiation of mouse and human myoblasts through a CB1-dependent mechanism (Iannotti et al. 2013).
Skeletal muscle development is a highly controlled, multifactorial process involving the coordinated regulation of a large number of genes that results in proliferating myoblast cells leaving the cell cycle, and subsequently fusing into ordered arrays of large multinucleated myotubes which further differentiate into mature muscle fibres (Iannotti et al. 2010).
Changes in the expression and functional activation of various classes of ion channels seem to be associated with the myoblast to myotube transition (Cooper, 2001).
Mouse C2C12 cells may be used in an experimental model for in vitro skeletal myogenesis, this method can be used to determine the potential effects of compounds on the skeletal muscle cell differentiation process.
A further model using human satellite cells can also be used as an in vitro test to study cell differentiation. Satellite cells are precursors to skeletal muscle cells, able to give rise to differentiated skeletal muscle cells. Upon activation, satellite cells can re-enter the cell cycle to proliferate and differentiate into myoblasts.
A myocyte is the type of cell found in muscle tissue, these are long, tubular cells that develop from myoblasts to form muscles in a process known as myogenesis. There are various specialized forms of myocytes: cardiac, skeletal, and smooth muscle cells, with various properties.
There are several different types of degenerative skeletal muscle diseases of which Duchenne muscular dystrophy (DMD) is by far the most common. Other types of degenerative skeletal muscle disease include: Becker muscular dystrophy (BMD); Congenital muscular dystrophy; Distal muscular dystrophy; Emery-Dreifuss Muscular Dystrophy; Facioscapulohumeral muscular dystrophy (FSHD); Limb-girdle muscular dystrophy (LGMD); Myotonic muscular dystrophy; and Oculopharyngeal muscular dystrophy.
DMD generally affects only boys (with extremely rare exceptions), becoming clinically evident when a child begins walking. By age 10, the child may need braces for walking and by age 12, most patients are unable to walk.
The life span of DMD patients ranges from 15 to 51. In the early 1990s, researchers identified the gene for the protein dystrophin which, when absent, causes DMD. The amount of dystrophin correlates with the severity of the disease (i.e., the less dystrophin present, the more severe the phenotype).
Since the gene is on the X chromosome, this disorder affects primarily males, and females who are carriers have milder symptoms. Sporadic mutations in this gene occur frequently, accounting for a third of cases. The remaining two-thirds of cases are inherited in a recessive pattern.
Dystrophin is part of a complex structure involving several other protein components. The “dystrophin-glycoprotein complex” helps anchor the structural skeleton (cytoskeleton) within the muscle cells, through the outer membrane (sarcolemma) of each cell, to the tissue framework (extracellular matrix) that surrounds each cell. Due to defects in this assembly, contraction of the muscle leads to disruption of the outer membrane of the muscle cells and eventual weakening and wasting of the muscle.
Glucocorticoids, more precisely prednisone and deflazacort, are the main drug treatments for DMD. They have been used for over two decades and are the only medications that have been shown to increase muscular strength.
As glucocorticoids are anti-inflammatory and immunosuppressant long term use of these compounds can result in many damaging side effects. Immunosuppression is a major problem in long term users of glucocorticoids which in turn can mean that a patient's immune system is less functional leaving them prone to serious infections. In addition wound healing requires a certain amount of inflammation and this can be delayed during glucocorticoid therapy.
Glucocorticoids can also raise blood sugar which in turn can diabetes mellitus. Calcium absorption may be suppressed and osteoporosis may result. Muscle atrophy can also occur with long-term glucocorticoid therapy.
In order to treat degenerative skeletal muscle diseases such as DMD it is important for a medication to enable satellite cell differentiation into myoblasts and myoblast differentiation into myotubes. It is known that CB1 activation also stimulates myoblast proliferation and therefore endocannabinoids, depending on “when” and “where” they act, can both inhibit and stimulate muscle formation by affecting cell cycle and plasticity.
Phytocannabinoids may act directly or indirectly at the CB1 receptor to counteract the effects of its activation. The phytocannabinoids may also counteract inflammatory responses such as those that occur in DMD. These inflammatory responses significantly worsen the consequences of impaired muscle differentiation, thus reducing the life expectancy of DMD patients. Additionally the phytocannabinoids may inhibit endocannabinoid inactivation.
It has been found that phytocannabinoids, in particular the phytocannabinoids CBD and CBDV are effective at enabling satellite cell differentiation into myoblasts and myoblast differentiation into myotubes. This was surprising as these compounds are not CB1 active compounds